The nitrogen oxides, NO and NO₂, are generically termed NO_x. NO_x plays a central role in tropospheric as well as in stratospheric chemistry.

There are several reasons why an improved knowledge of the global stratospheric and especially tropospheric distribution of NO_x is important:

• NO_x and volatile organic compounds (VOC) are emitted in the tropospheric boundary layer in large quantities from human activities such as traffic and industry. In the summer months this mixture is capable of photochemical smog production.
• The chemical budget of ozone in the troposphere is largely determined by the concentration of NO_x.
• Our knowledge of ozone distribution and its budget is limited by a lack of information about the concentrations of NO and NO₂ in the troposphere.
• NO_x contributes to acid rain, and with decreasing SO₂ emissions its role in causing acidity is increasing.
• In the stratosphere, NO_x has a large impact on ozone levels. On the one hand it can destroy ozone catalytically, but on the other hand it also forms stable reservoir species with halogen oxides, thereby reducing ozone depletion.

This section presents the role of NO₂ in terms of the following topics:

• The types and characteristics of nitrogen compounds in the atmosphere.
• The sources of NO_x in the atmosphere, and their altitude and spatial distribution.
• The reactions of tropospheric NO_x.
• The reactions of stratospheric NO_x.
• The task of measuring NO_x in the atmosphere.
• A case study of the remote sensing from space of emissions caused by biomass burning.